Age‐Dependent Changes in the Subcellular Distribution of Rat Brain μ‐Opioid Receptors and GTP Binding Regulatory Proteins

: The relative subcellular distributions of μ‐opioid receptors and guanine nucleotide binding regulatory proteins (G proteins) in 1‐day‐old (P1) and adult rat forebrain were compared. Light membranes (LMs) were resolved from heavy membranes (HM) by sucrose density gradient centrifugation. Marker enzyme analyses indicated that LMs contained most of the endoplasmic reticulum and Golgi complexes, whereas HMs were enriched in plasma membranes. Binding distribution and properties of μ‐opioid sites were assessed using [3H][d‐Ala2,Me‐Phe4,Gly‐ol5]enkephalin. P1 LMs possessed 43% of die total μ‐opioid binding detected compared to 16% in the adult. Although NaCl inhibited μ binding in LMs to a greater extent than in HMs, age‐dependent differences were not observed. P1 LM μ binding possessed greater sensitivity to 5′‐guanylylimidodiphosphate than their adult counterpart. Moreover, P1 LMs contained more Gocr. protein than P1 HMs or adult LMs, as demonstrated by immunoblotting with antisera against Gocr after one‐or two‐dimensional gel electrophoresis. These results suggest that P1 LMs contain a greater proportion of newly synthesized intracellular μ sites than adult LMs.